President Barack Obama vowed Wednesday that the United States would not be intimidated by the beheading of a second American reporter but acknowledged the fight against the jihadists would take time.
Obama pledged that justice would be done to the Islamic State (IS) killers of 31-year-old reporter Steven Sotloff, wherever they hid and however long it took.
But he warned that eliminating the threat posed to the region by the group from its bases in Iraq and Syria would take time.
IS posted video footage on the Internet of Sotloff's beheading, confirmed as authentic by Washington, which sparked outrage around the world.
It said the journalist's killing, which comes on the heels of the beheading last month of another US reporter, James Foley, was in retaliation for expanded US air strikes against its fighters in Iraq during the past week.
It warned a British hostage would be next unless London backs off from its support for Washington's air campaign.
But Prime Minister David Cameron said that Britain too refused to be cowed, while Foreign Minister Philip Hammond said that British air strikes were not being ruled out.
Obama said the whole world had been repulsed by the barbarism of Sotloff's murder but "we will not be intimidated".
"Those who make the mistake of harming Americans will learn that we will not forget and that our reach is long and that justice will be served," he said.
Obama said Washington was determined to halt the IS threat but warned it would depend on close cooperation with partners in the region.
He has previously admitted that his administration has yet to develop a comprehensive strategy for tackling IS on both sides of the Iraq-Syria border.
Washington has ruled out any air strikes for now against IS fighters on the Syrian side, where they hold a swathe of the east.
In violence on Wednesday, 10 children were among 16 people killed in an IS-controlled area of eastern Syria, a monitoring group said.
The Syrian Observatory for Human Rights said a regime air raid hit a bus, but state television blamed the jihadists.
In northern Syria, several historic buildings, including a colonial-era structure, have been blown up in the city of Aleppo, reports said Wednesday.
State media said rebels in Aleppo destroyed "several old buildings on Tuesday night by placing explosives in a tunnel". The Observatory said the regime was behind the blasts.
- 'Despicable act' -
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The British prime minister said the beheading video depicted an "absolutely disgusting, despicable act" and chaired a meeting of security chiefs to discuss how to tackle the IS threat.
The masked executioner in the video spoke with a London accent and claimed to be the same man, confirmed by UK security services as a Briton, who beheaded Foley.
"I'm back, Obama, and I'm back because of your arrogant foreign policy towards the Islamic State," the black-clad jihadist says, wielding a combat knife.
"So just as your missiles continue to strike the necks of our people, our knife will continue to strike the necks of your people," he declares, before reaching round to cut his captive's throat.
At the end of the five-minute recording, the militant threatens another captive, identified as Briton David Cawthorne Haines.
"We take this opportunity to warn those governments that enter this evil alliance of America against the Islamic State to back off and leave our people alone," he says.
Britain has maintained a media silence about the kidnapping of aid worker Cawthorne Haines and there were few immediate details about when or how he was abducted.
"We have to deal with (IS) on the basis of the wider threat that they pose to the British public as well as this individual," the British foreign minister said.
"If we judge that air strikes could be beneficial, could be the best way to do that, then we will certainly consider them but we have made no decision to do so at the moment."
Britain has so far only carried out reconnaissance flights in support of the US air campaign from its base in Cyprus.
- 'Brave and talented' -
In a statement, the Sotloff family, who live in Miami, said: "The family knows of this horrific tragedy and is grieving privately. There will be no public comment from the family during this difficult time."
Israeli media reported that the family was Jewish and that Sotloff himself held joint US-Israeli nationality but IS made no mention of either in its video.
Sotloff's former employers at Time and Foreign Policy paid tribute to a man widely respected for his intrepid reporting in Syria and the wider region, including a previous stint in Libya.
UN Secretary General Ban Ki-moon said the world was outraged at the beheading.
Hours after the posting of the video, the White House announced that Obama had authorised about 350 more US troops to beef up security at US diplomatic facilities and protect personnel in Baghdad.